A wide variety of mental and physical processes are controlled or influenced by neural activity in particular regions of the brain. For example, the neural-functions in some areas of the brain (i.e., the sensory or motor cortices) are organized according to physical or cognitive functions. There are also several other areas of the brain that appear to have distinct functions in most individuals. In the majority of people, for example, the areas of the occipital lobes relate to vision, the regions of the left interior frontal lobes relate to language, and the regions of the cerebral cortex appear to be consistently involved with conscious awareness, memory, and intellect.
Many problems or abnormalities with body functions can be caused by damage, disease and/or disorders in the brain. Effectively treating such abnormalities may be very difficult. For example, a stroke is a very common condition that damages the brain. Strokes are generally caused by emboli (e.g., obstruction of a vessel), hemorrhages (e.g., rupture of a vessel), or thrombi (e.g., clotting) in the vascular system of a specific region of the brain, which in turn generally cause a loss or impairment of a neural function (e.g., neural functions related to facial muscles, limbs, speech, etc.). Stroke patients are typically treated using various forms of physical therapy to rehabilitate the loss of function of a limb or another affected body part. Stroke patients may also be treated using physical therapy plus drug treatment. For most patients, however, such treatments are not sufficient, and little can be done to improve the function of an affected body part beyond the limited recovery that generally occurs naturally without intervention.
The neural activity in the brain can be influenced by electrical energy that is supplied from a waveform generator or other type of device. Various patient perceptions and/or neural functions can thus be promoted or disrupted by applying an electrical current to the cortex or other region of the brain. As a result, researchers have attempted to treat various neurological conditions using electrical or magnetic stimulation signals to control or affect brain functions.
Neural activity is governed by electrical impulses or “action potentials” generated in and propagated by neurons. While in a quiescent state, a neuron is negatively polarized, and exhibits a resting membrane potential that is typically between −70 and −60 mV. Through electrical or chemical connections known as synapses, any given neuron receives from other neurons excitatory and inhibitory input signals or stimuli. A neuron integrates the excitatory and inhibitory input signals it receives, and generates or fires a series of action potentials in the event that the integration exceeds a threshold potential. A neural firing threshold may be, for example, approximately −55 mV. Action potentials propagate to the neuron's synapses, where they are conveyed to other neurons to which the neuron is synaptically connected.
A neural stimulation signal may comprise a series or train of electrical or magnetic pulses that deliver an energy dose to neurons within a target neural population. The stimulation signal may be defined or described in accordance with stimulation signal parameters including pulse amplitude, pulse frequency, duty cycle, stimulation signal duration, and/or other parameters. Electrical or magnetic stimulation signals applied to a population of neurons can depolarize neurons within the population toward their threshold potentials. Depending upon stimulation signal parameters, this depolarization can cause neurons to generate or fire action potentials. Neural stimulation that elicits or induces action potentials in a functionally significant proportion of the neural population to which the stimulation is applied is referred to as supra-threshold stimulation; neural stimulation that fails to elicit action potentials in a functionally significant proportion of the neural population is defined as sub-threshold stimulation. In general, supra-threshold stimulation of a neural population triggers or activates one or more functions associated with the neural population, but sub-threshold stimulation by itself fails to trigger or activate such functions. Supra-threshold neural stimulation can induce various types of measurable or monitorable responses in a patient. For example, supra-threshold stimulation applied to a patient's motor cortex can induce muscle fiber contractions.
While electrical or magnetic stimulation of neural tissue may be directed toward producing an intended type of neural activity, such stimulation may result in unintended collateral neural activity. In particular, neural stimulation for treating a condition can induce seizure activity or other types of collateral neural activity. It will be appreciated that such collateral neural activity is undesirable and/or inconvenient in a neural stimulation situation.
Seizure activity may originate at a seizure focus, which is typically a collection of neurons (e.g., on the order of 1000 neurons) exhibiting a characteristic type of synchronous firing activity. In particular, each neuron within a seizure focus exhibits a firing response known as a paroxysmal depolarizing shift (PDS). The PDS is a large magnitude, long duration depolarization that triggers a neuron to fire a train or burst of action potentials. Properly functioning feedback and/or feed-forward inhibitory signaling mechanisms cause afterhyperpolarization through which the neuron's membrane potential returns to a hyperpolarized state below its firing threshold. Following afterhyperpolarization, the neuron may undergo another PDS.
Afterhyperpolarization limits the duration of the PDS, thereby helping to ensure that synchronous neural firing activity remains localized to the seizure focus. Inhibitory feedback signaling provided by neurons surrounding a seizure focus, commonly referred to as “surround inhibition,” is particularly important in constraining seizure activity to the seizure focus. In the event that inhibitory signaling mechanisms fail and/or are unable to overcome or counter PDS activity, neurons within the seizure focus recruit other neurons to which they are synaptically coupled into their synchronous firing pattern. As a result, synchronous firing activity spreads beyond the seizure focus to other areas of the brain. This can lead to a cascade effect in which seizure activity becomes increasingly widespread, and accompanying clinical manifestations become increasingly significant.
In view of the foregoing, it may be important to detect and/or respond to seizure activity. Various systems and/or devices directed toward treating neurological conditions exist, including those capable of detecting and responding to particular types of neurological events. For example, some neural stimulators attempt to treat involuntary motion disorders such as Parkinson's disease by applying stimulation signals to the thalamus or other area of a patient's brain. As another example, U.S. Pat. No. 6,134,474 describes an implantable device capable of detecting a neurological event, such as seizure activity, and generating a responsive electrical signal intended to terminate the detected event. Additionally, European Patent Application Publication EP1145736 describes an implantable device capable of detecting electrical activity in the brain; applying a nonresponsive signal to reduce the likelihood of a seizure occurring; and applying a responsive signal in the event that epileptiform activity is detected.
Unfortunately, present neural stimulation systems and methods fail to automatically detect and/or respond to seizure activity or other collateral neural activity induced in association with and/or as a result of neural stimulation procedures directed toward purposes other than epileptic seizure management. In particular, conventional neural stimulation systems fail to automatically detect seizure activity induced by neural stimulation procedures directed toward patient neural function rehabilitation and/or enhancement, or modulation of patient sensory perceptions.